Preliminary Definitions: As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires:                The adjective “reconfigurable” as applied to a communication network shall mean that the communication network has more than two nodes, and that connections among the nodes can be “changed.” “Changing a connection” among nodes of a network of at least three nodes is defined to mean permuting the order of nodes through which a communication traverses nodes of the network. Thus, the institution of a new communication link as part of a network constitutes an example of “changing a connection.” Merely changing a communication protocol, where there has been no change in order of nodes, does not constitute “reconfiguration,” as the term is defined herein.        The term “repositionable” shall refer to the fact that the relative position in of the communication nodes can be moved in coordinate space, e.g., to a different location in a city, or a different location on a battlefield.        “Free-space” shall refer to propagation of an electromagnetic wave that is unguided is the sense that no structure imposes boundary conditions on the field comprising the wave.        “Free-space distribution” of information, or of a quantum signal, as the case may be, shall refer to transfer of that information, or, mutatis mutandis, of that quantum signal, via an electromagnetic wave subject to free-space propagation over at least a portion of the path over which the information or quantum signal is distributed. (See below for the definition of “quantum signal.”)        The term “hover” shall mean to maintain a substantially fixed vertical relationship with respect to a specified locus on the surface of the Earth. It is to be understood that the specified locus may be in motion, in which case a hovering object will track the motion of the specified locus.        The term “vertically hovering” is synonymous with “hovering.”        
Wireless transmission of many types of sensitive data to be transmitted wirelessly creates a problem of security with respect to access to that information. This is particularly important when the transmitted data includes HIPAA-protected medical data, bank data, military drone technology and military data. As an example of compromised security, Predator military drones have reportedly been hacked. The creation of unbreakable encryption technology is thus vital to protect sensitive data.
Quantum cryptography has been determined to be an unbreakable encryption technology, when properly implemented. Quantum cryptography has thus far seen a number of demonstrations, either using a fiber optic channel (as might potentially be networked, although most of the demonstrations have been single dedicated fiber links), or using a free-space optical channel. The latter has been demonstrated in laboratories, across cities, and between two of the Canary Islands (at a distance of 144 km), as described by Ursin et al., “Entanglement-based quantum communication over 144 km,” Nature Phys., vol. 3, pp. 481-486 (2007), which is incorporated herein by reference.
The drawback of each of these is that the sending and receiving nodes are fixed, and generally not particularly portable. The participle “fixed,” as used herein to describe a node of a network relative to another node, relates to the relative geometrical position of one node with respect to the other node, in three-dimensional position space. For example, in the demonstrations using telescopes, these are rather large, and must be very carefully aligned with each other in order to permit the optical link. For the fiber optic links, one must usually have a dedicated fiber optic line (in any event, there needs to be a direct optical path from sender to receiver). This is fine if there are already optical fibers in place, but in many cases that would not be the situation.
A general review of secure quantum key distribution may be found in Lo et al., “Secure quantum key distribution.” Nature Photonics, vol. 8, pp. 595-604 (2014), which is incorporated herein by reference. All free-space distribution of quantum keys that has ever been suggested has been between two nodes that share a line of sight. Examples include:                fixed telescopes at each of two nodes, as discussed, for example, by Buttler et al., “Daylight quantum key distribution over 1.6 km,” Phys. Rev. Lett., vol. 84, 5652 (2000), incorporated herein by reference, and subsequently by others.        a fixed telescope to a moving truck: Bourgoin et al., “Free-space quantum key distribution to a moving receiver,” Opt. Exp., vol. 23, pp. 33437-47 (2015); and        plane to ground: Nauerth et al., “Air-to-ground quantum communication,” Nature Photonics, vol. 7, pp. 382-386 (2013).All of the foregoing references are incorporated herein by reference.        
All free-space QKD systems that have been either implemented or proposed have entailed a line of sight between at least two nodes of a network that, for at least some period of time, is either direct or fixed or both. For example, U.S. Pat. No. 5,966,224 (to Hughes et al.) contemplates two fixed earth stations in optical communication with a commonly accessed satellite.
While the paper, Qi et al., “Free-space reconfigurable quantum key distribution network,” 2015 IEEE International Conference on Space Optical Systems and Applications (ICSOS), pp. 1-6, (Oct. 16, 2015) (hereinafter “Qi (2015), incorporated herein by reference) suggests the application of quantum key distribution (QKD) in a mobile network using an untrusted network relay; however, in that paper the term “reconfigurable” refers to the capacity to readily switch between a highly secure measurement-device-independent (MDI) QKD system and a highly efficient decoy state, the Bennett and Brassard quantum cryptography protocol BB84. Qi (2015) does not, however, contemplate how secure communications might be established, in practice, between two parties, of which the first is free to move with respect to the second.
All secure communications links in the prior art have required that either a free-space link or a fiber link exist either between two nodes or between both of the nodes and a common relay. Methods for overcoming that limitation would be of great value.
Certain embodiments of the invention described below are directed toward solving the problem of creating an unbreakable encryption technology where the spatial relationship between two or more parties is reconfigurable. Properly implemented, it may enable quantum cryptographic links to be readily established, e.g., on the battlefield, between branches of a bank, between two drones, or even between residential users and a central communication hub.